1. Field of the Invention
The present invention relates to a light beam scanning device in which scanning speed irregularities of a light spot on a scanning plane are corrected, and an effective length of scanning line can be finely adjusted.
2. Description of the Prior Art
A variety of light beam scanning devices have been developed for recording data on a recording material which is placed on a plane (hereinafter referred to as the "scanning plane") which is scanned with a light spot. These devices also read data from an original which is placed on the scanning plane, and they generally employ vibrating mirrors, such as galvanometers or rotary multi-mirrors, as their light beam deflectors. In these devices, it is desirable that the light spot scan the scanning plane at a constant speed because, if the scanning speed is irregular and non-constant, the pattern which is recorded or reproduced is distorted. For instance, in the case where the galvanometer is used, the scanning speed is considerably irregular when the accuracy of forming a sawtooth drive signal and the response accuracy or repetition accuracy of the galvanometer are taken into consideration. The irregularity is, in general, increased as the speed is increased.
On the other hand, when a rotary multi-mirror is used as the light beam deflector instead of a galvanometer, it is considerably difficult to rotate the mirror at a constant speed. Therefore, the scanning speed is irregular.
Heretofore, scanning speed irregularities have been substantially corrected by a method in which a grating pattern, which has transparent and opaque parts arranged alternately with a certain period in the scanning direction, is placed in a plane which is substantially equivalent to the scanning plane at which data are recorded or read. A photoelectric signal which is obtained from the grating pattern is then utilized as the scanning reference signal.
FIGS. 1, 2 and 3 show examples of a light beam scanning device incorporating this method. In these figures, reference numeral 1 designates a light beam deflector; reference numeral 2 illustrates a scanning lens; reference numeral 3 shows a scanning plane on which a recording material or an original to be read is placed; reference numeral 5 designates a grating pattern which is located on a grating plate; reference numeral 6 illustrates a condenser lens; reference numeral 7 shows a photo-detector; reference numeral 8 designates a first laser beam (indicated by a solid line) for scanning a recording material or original; and reference numeral 9 illustrates a second laser beam (indicated by the broken line) for scanning the grating pattern.
In FIG. 1, both the first and second laser beams 8 and 9 are coaxially applied to the vibrating mirror 1. After the beams 8 and 9 have been deflected by the vibrating mirror 1, these beams are separated from each other by a beam separating mirror 4 so that the first laser beam scans the scanning plane 3 on which a recording material or an original is placed, while the second laser beam scans the grating pattern 5.
In FIG. 2, the first laser beam 8 and the second laser beam 9 are applied to the vibrating mirror 1 at different incident angles; more specifically, the beams 8 and 9 are applied to both surfaces of the vibrating mirror 1, respectively. The first laser beam 8 is deflected by the vibrating mirror 1 and passes through a first scanning lens 2 in order to scan the scanning plane 3 on which a recording material or an original is placed. The second laser beam 9 is deflected by the rear surface of the vibrating mirror 1 and passes through a second scanning lens 2' in order to scan the grating pattern 5.
The performance of the apparatus shown in FIG. 3 is excellent, and this apparatus is relatively inexpensive to manufacture. In this apparatus, the first laser beam 8 is deflected by the vibrating mirror 1, and it then passes through the scanning lens 2 to scan the scanning plane 3 on which a recording material or an original is placed. The second laser beam 9 passes through a focusing lens 10 and is deflected by the rear surface of the vibrating mirror 1. The deflected laser beam 9 then scans a grating pattern 5 which is arranged in a deflection plane along a circumference which has the beam deflecting point as its center. Thus, the focusing lens 10 acts to focus the laser beam 9 on the grating pattern 5.
In any one of the methods described with reference to FIGS. 1, 2 and 3, the laser beam which is modulated by the grating pattern 5 is collected at the photo-detector 7 by the condenser lens so that scanning time is obtained with the output photoelectric signal of the detector 7 as a reference. The scanning lens 2 and 2' are generally f.theta. lenses, and in this case, the grating pattern 5 has transparent and opaque parts which are arranged alternately at a predetermined period. With a provision of such a grating pattern, scanning irregularities of a light spot on a scanning plane are corrected, since a signal clock timing of the light spot is controlled in timed relation to positions of the light spot. In addition, the length of the scanning line (scanning length) on the scanning plane is determined by the length of the grating pattern in the scanning direction.
In each of the above-described, conventional light beam scanning devices, the period of the grating pattern 5 is fixed, and, therefore, only one kind of photoelectric signal is obtained as the scanning reference signal, and the scanning length on the scanning plane is solely determined. This is disadvantageous in the case where it is necessary to finely adjust the scanning length. A typical example of a case in which the scanning length must be finely adjusted is the case in which, like the present invention, the light beam scanning is applied to laser printers or laser COM's (Computer Output Microfilmer) .
For instance, in the case of a laser printer, a scanning laser beam is used to print output data from an electronic computer on a sheet on which forms, such as tables and frames, have been printed. Alternatively, the laser beam is used to print the output data on a blank sheet, together with a form which is exposed through a separately provided optical system. In either case, the timing of the laser beam scanning should be determined so that the data size is in agreement with the required, predetermined form size. The accuracy required in the case of a laser printer is 1/3000 (.about.0.03%) if the number of points to be resolved on a scanning line is 3000 points, and an allowable error is one point in the number of resolution points. Thus, extremely high accuracy is required.
In order for the conventional technique to achieve such a high accuracy, the focal length of the f.theta. lens, the period of the grating pattern and the adjustment of the entire optical system must all be extremely accurate. Accordingly, the light beam scanning device is necessarily very expensive when such high accuracy is required.